1. Field of the Invention
The invention relates to a tube film blowing head with one outer and one inner nozzle ring between which a ring chamber is formed extending in a conical or parabolic manner towards the circular nozzle gap and of which the inner nozzle ring, which is provided with a central bore for exchanging the pressurized air, is connected by means of a screw connection to a housing piece or a central cone piece of the blowing head having a bore that is aligned to the central bore.
2. Description of the Related Art
Such a known tube film blowing head, which is shown in FIGS. 8 and 9 of the drawings, comprises a central cylindrical cone piece 1, which is connected to a disc-shaped base 2 by way of a ring stage. The central cylindrical cone piece 1 is surrounded by a ring-shaped housing piece 3 that is supported in a sealing manner on the ring stage of the disc-shaped base by way of its lower, flat face. The ring-shaped housing piece 3 is connected to the disc-shaped base 2 by way of screws, which are not shown. A ring chamber is formed between the central cone piece 1 and the ring-shaped housing piece 3 surrounding it, which ring chamber is provided with the conventional spiral-shaped channels to the supply of the melt which, in its flow in the direction of the circular nozzle gap 4, increasingly flows away in the axial direction over the walls limiting the channels. The central cone piece 1 is provided with a central blind bore 5 in which protruding tube 6 is arranged. On the tube 6, a diagonally running tube 7 is arranged that passes through the lower region of the cone piece 1 and the disc-shaped base 2 in the manner shown. Diagonally running bores or lines 8 empty into the lower region of the blind bore 5 of the cone piece 1. Through the lines 8, pressurized air is supplied to the ring channel 9, which is formed between the blind bore 5 and the outer jacket of the tube 6, which pressurized air serves to extend and to cool the film tube that has been extruded and blown up to a tube bubble. The cooling air blown in then arrives in the upper, free end of the tube 6 and is removed by way of the tube 7 in a known manner. The disc-shaped base is provided with diagonally running bores 10, which empty into the beginning region of the spiral-shaped channels 11, 12. The bores 10, which run together, are connected to the line 13 supplying the melt in a known manner. The inner ring 15 of the extrusion nozzles is placed on the upper, flat, ring-shaped face of the cone piece 1 and is screwed onto the housing piece 1 by means of screws 16. The screws 16 penetrate the inner nozzle ring 15 in axial bores that are arranged in the manner shown in FIG. 8 at equal circumferential distances from one another in a circle 17 that is concentric to the center axis. The tension screws 16 are screwed into aligning threaded bores of the cylindrical cone piece 1. The inner nozzle ring 15 is surrounded by an outer nozzle ring 18 that is also connected to the ring-shaped housing piece 3 in a corresponding manner by means of screw connections. Between the inner and outer nozzle rings, a ring chamber 19 is formed whose lower end is adjacent to the ring chamber formed between the cone piece 1 and the ring-shaped housing piece 3. The ring chamber 19 formed between the two nozzle rings, which guides the melt to the nozzle gap 4, widens in the shape of a cone towards the nozzle gap 4 beginning from its lower end. This has the result that a higher hydraulic pressure is acting on the inner nozzle ring 15, which attempts to lift the inner nozzle ring from the cylindrical cone piece 1.
In order to achieve a sufficiently firm connection of the inner nozzle ring to the cone piece 1, as can be seen in FIG. 8, as many screws 16 as possible are provided for the purpose of connecting these two parts. In spite of the relatively high number of screws 16, together they have only one cross-section, which does not guarantee a sufficiently firm connection or creates a sufficiently firm connection only at great expense.
The object of the invention is therefore to create a screw connection between the inner nozzle ring and the center cone piece that is firm and simple to produce.
This object is attained according to the present invention with a tube film blowing head with one outer and one inner nozzle ring between which a ring chamber is formed extending in a conical or parabolic manner toward the circular nozzle gap. The inner nozzle ring is provided with a central bore for exchanging the pressurized air and is connected by means of a screw connection to a housing piece and/or a central cone piece of the blowing head having a bore that is aligned to the central bore. According to the present invention, the central bore of the inner nozzle ring is penetrated by an individual tension element also having an axial bore, by means of which the inner nozzle ring can be pressed against the central cone piece.
In accordance with a preferred embodiment of the invention, the inner nozzle ring is connected to the central cone piece by means of a threaded bolt penetrating its central bore and provided with an axial bore in that the threaded bolt is screwed into the bore of the cone piece that is embodied as a threaded bore.
Alternately, the inner nozzle ring is connected to the central cone piece by means of a threaded bolt provided with an axial bore in that the screw head of the threaded bolt is set into the cone piece and in that, in the region of the inner nozzle ring, a nut is screwed onto an outer thread of the threaded bolt and in that several tension screws for the purpose of tensing the inner nozzle ring relative to the central cone piece are arranged in the nut in an evenly spaced manner circumferentially.
The threaded bolt provided according to the invention can be embodied with such a large outer diameter that, in spite of the central bore penetrating it, it has a large cross sectional area that guarantees a high degree of firmness.
The threaded bolt according to the invention can be connected to the inner nozzle ring in a different manner and support itself on the inner nozzle ring. The threaded bolt can be provided with a head that is supported on the inner edge region of the inner ring.
If, because of the size of the threaded bolt, difficulties arise in pulling a head fixed to it against the inner nozzle ring in a firm manner, it is possible in another embodiment of the invention to provide for the head, which is embodied in the shape of a flange, to be provided with threaded bores that are concentric and parallel to the center line of the threaded bolt, into which threaded bores the screws supported on the inner ring may be screwed.
In accordance with another preferred embodiment, it is provided for the threaded bolt to be provided with a thread on its upper end as well onto which a nut is screwed that is supported on the inner nozzle ring.
The threaded bolt can also be provided with a thread on its upper end onto which a nut is screwed that is not supported directly on the inner nozzle ring, but rather is provided with threaded bores concentrically and parallel to the center line of the threaded bolt, into which threaded bores the screws supported on the inner ring may be screwed.
In accordance with another embodiment of the invention, whose use is particularly useful for a multi-layer film blowing head, it is provided for the inner ring to be provided with a recess forming a ring stage into which the head of the threaded bolt grips.